The invention relates to a process and a device for alignment of sheet material during its transport in orthogonal directions in its conveyor plane before processing in a machine which processes sheet material.
U.S. Pat. No. 5,322,273 discloses a sheet alignment device. This device for alignment of a sheet moving along an essentially flat transport path enables alignment of a moving sheet in a plurality of orthogonal directions, for example transversely to the transport path, in the direction of the transport path, and to eliminate skewed positions. The sheet alignment device has a first roller arrangement with a first pressure roller which is supported such that it can turn around one axis which lies in a plane which extends parallel to the plane of the transport path and runs essentially at a right angle to the direction of sheet transport along the transport path. A second roller arrangement has a second pressure roller which is supported such that it can turn around one axis which lies in a plane which extends parallel to the plane of the transport path and runs essentially at a right angle to the direction of sheet transport along the transport path. There is a third roller arrangement which has a third pressure roller which is supported such that it can turn around one axis which lies in a plane which extends parallel to the plane of the transport path and runs essentially at a right angle to the direction of sheet transport along the transport path. The third roller arrangement which can turn around one axis which lies in a plane which extends parallel to the plane of the transport path and runs essentially at a right angle to the direction of sheet transport along the transport path can be moved along its axis of rotation in the direction which runs transversely to the transport path. Finally, there is a control means which is dynamically connected to the first and the second and the third roller arrangement and selectively controls the rotation of the first and second roller arrangement in order to align the front edge of a sheet moving in the direction of sheet transport along the transport path into the position which is at a right angle to the direction of sheet transport. The control means furthermore controls the rotation and the transverse motion of the third roller arrangement in order to align the moving sheet in the direction which runs transversely to the direction of sheet transport and in the direction in which the sheet is moving along the transport path.
The sheet alignment device known from U.S. Pat. No. 5,322,273 enables the required alignment accuracies to be satisfied only to a limited degree. To achieve the required alignment accuracies, extensive modification of the sheet alignment device of the prior art is necessary, which modification does not seem economical.
In sheet-processing printing presses which work using the offset principle, the sheets are conveyed on the feed table in a ragged arrangement before they are aligned on the side and pull-type lay marks which are provided in the plane of the feed table. After completed alignment of the sheet material it is transferred in the aligned state to a pre-gripper which accelerates the sheet material to the press speed and transfers it to the sheet-guiding cylinder which is located downstream of the pre-gripper means. Other alignment concepts generally use cylindrical rollers with a rubber coating which can be held on their core. If with this configuration alignment of the sheet material is carried out during its feed by changing the speed between the left and right roller which grip the sheet material, the sheet material undergoes rotation around a pivot which is located on the stationary roller or during feed is located outside the roller with lower rpm or between the two rollers.
When the sheet material is being aligned by segmented rollers, a segment path of less than 360 degrees is available for the correction motion by the alignment elements if they are made as segmented rollers. If the sheet material is aligned in the conveyor direction and transversely to the conveyor direction by alignment elements which sit on an axle, the available segment path of  less than 360 degrees is divided among the two alignment functions. If the alignment process takes place in start-stop operation, the necessary segment path is minimal. Since however here the continuous feed of sheet material is interrupted, in front of the alignment unit either there can be a paper reservoir, for example in the form of staggering of the sheets, or a relatively large distance can be maintained between the individual copies of the sheet material, by which there the process speed of the machine which processes the sheet material is limited. In the alignment process of the sheet material by means of a segmented roller, the problem necessarily arises that the alignment motion is limited to the maximum available segment periphery. An increase in the size of the periphery of the alignment element in the form of a segmented roller by increasing the diameter as the positioning accuracy on the segment periphery remains the same would entail a higher angular resolution of the pertinent actuator and thus follow-up costs, which is worth avoiding.
The object of the invention in view of the approach known from the prior art and the indicated technical problem is to undertake the correction movement necessary for alignment of the sheet material during its transport.
The advantages which can be achieved with the approach in the invention are mainly that by dividing the alignment functions between an alignment function in the conveyor direction of the sheet material and an alignment function perpendicular to the conveyor direction of the sheet material, a complete segment periphery of 360 degrees is available for each individual alignment function. Thus the alignment path can be increased for the individual functions with the resolution remaining the same. A uniform resolution allows retention of the segment periphery; higher angular resolution which is necessary due to the increase of the segment periphery and thus higher resolution of the pertinent actuator can be omitted. Another advantage lies in that the motion sequences take place in the conveyor direction of the sheet material and transversely thereto, independently of one another. Therefore the sheet material need no longer be stopped or braked for its alignment in at least two planes, but the correction movements can be superimposed using the complete peripheral surfaces of the alignment elements on the process speed, i.e. the feed rate of the sheet material to the processing machine which processes sheet material. Thus, the feed rate can be increased since braking processes are not necessary. Furthermore, a paper reservoir unit which represents additional cost can be omitted.
In another embodiment of the process in the invention, on the alignment elements for alignment of the sheet material their entire peripheral surface can be used. Thus, reliable alignment of the sheet material is ensured even at the highest feed rates. The alignment elements can be triggered independently of one another using the process proposed as claimed in the invention, especially via separate drives. The alignment functions on the sheet material can take place, viewed in its conveyor direction, horizontally in succession, thus for example first of all alignment in the conveyor direction, subsequent to which alignment can then take place transversely to the conveyor direction.
By using the periphery which extends on the segmented rollers for example as a three-quarters circle, an increase in diameter of the segmented rollers and a concomitant increase of the resolution of the actuators can be avoided. Thus higher costs do not arise in alignment of sheet material with the process proposed as claimed in the invention.
Likewise, in the invention a device for alignment of sheet material is proposed where the alignment elements are driven via alignment of the sheet material in the conveyor direction or transversely thereto via drives which are independent of one another. By means of the alignment element drives which are independent of the feed drive of the sheet material, decoupling of the alignment processes from the feed motion and this superposition of the alignment function on the feed function can be guaranteed.
In one advantageous embodiment of the process proposed in the invention, for the individual function of alignment in the lengthwise direction of the sheet material and transversely thereto the complete segment periphery of the alignment element is available. The segment periphery, depending on the size of the interruption on the periphery of the segment, can be less than 360 degrees, preferably the peripheries on the segments can have a three quarters circular arc extension.
The alignment device proposed in the invention which comprises division of the respective alignment function in the conveyor direction of the sheet material and transversely thereto, can be implemented on feed means such as a feeder for sheet material and can be used to advantage on machines which process sheet material. These machines can be for example printing presses, digital printing units and also printing presses which print images digitally or directly.